Sephiroth... major spoilers that might change your view on who is the best villain!!!

[Raistlin]

If I step on a few ants, am I insane? There's a difference between "evil" and "insane"...

Yes, but take a good look at Sephiroth's character before and during the Nibelheim incident. He was calm, composed, possibly arrogant?
Less than an hour later: he's destroying the town, raving about freeing his "mother" and other such nonsense.

If that's not insane, I don't know what is.

[Doomgaze]

I think it was at LEAST a few days, I don't remember.

[Big D]

Well, sure; but it's a heck of a change of personality (and lifestyle) to happen in such a short space of time.
There's a sizeable grey area at the intersection of "eccentric", "determined", "zealous" and "totally frickin' off yer rocker"; but I feel that Sephiroth was quite firmly situated somewhere near the latter category. Now, you could argue that he wasn't really insane by reason that his more unusual notions might've been planted by Jenova, but that's all water under the bridge. He talked like a madman, he obsessed like a madman, he massacred cities and beheaded maternal figures like a madman. Insofar as the external observer is concerned, Sephiroth was buying up large in the 'La-La Land Real Estate'.
[q=PhoenixAsh]

That's up to debate...the simplest, and most likely answer is "game mechanics." However, that's possible evidence to show that Cloud may have been given Jenova cells prior to the Nibelheim incident.

Erm.... Wha???
[/q]There's some evidence; however it's very vague and extremely speculative. I don't personally believe it, but it's impossible to rule out. Raistlin is one of, if not the only advocate of the idea. I'm sure he'll be willing to fill you in. I could argue both sides... but I won't. More fun this way...

[Nevermore]

Let's review what happened: Sephiroth slashes Tifa and knocks her aside, then heads into Jenova's chamber. Zack follows, and is easily defeated and knocked out of the room. Cloud follows, taking Zack's Buster Sword. Now, Sephiroth has turned his attention to Jenova, and isn't watching behind him. Cloud runs in and stabs him with the buster sword. Now, Sephiroth leaves, injured, with Jenova's head. Cloud follows, and Sephiroth says "Don't push your luck", and impales Cloud with the Masamune. Cloud lifts him up by the blade and tosses him into the Lifestream. See, Sephiroth is ALSO severely injured at this point - that, combined with Cloud's anger (a limit break?) is why Cloud can chuck him over the edge.

Yeah, it's what I saw. Cloud probably wouldn't have had much of a chance were Sephiroth not turned around and concentrating on something else.

As for Cloud's mind giving him his memories..I thought I suggested that earlier..and I was wrong. Now it's right? Er..
I thought Cloud did all this on his own, prior to having thought his mind and the Jenova cells gave him Zack's identity.

[Raistlin]

Originally posted by Doomgaze
I think it was at LEAST a few days, I don't remember.

Well, depends on your take on it. If you're talking about from the time in the Reactor, then yeah, it was a day or two. However, Sephiroth spent that day or two in the Shinra Mansion, reading. Hard to tell at exactly what point he went nuts. So, I'd say a day, at most.

There's some evidence; however it's very vague and extremely speculative.

I know, which is why I rarely mention it.
It's just one of my "interesting theories" to explain a part in the game that isn't explained very well by anything else. Kinda like the Jenova-destroying-the-world one.

[§håd0w]

Originally posted by Yamaneko
Well, depends on your take on it. If you're talking about from the time in the Reactor, then yeah, it was a day or two. However, Sephiroth spent that day or two in the Shinra Mansion, reading. Hard to tell at exactly what point he went nuts. So, I'd say a day, at most.

Sephiroth 'locked' himself (as so its says) in the Shinra Basment. He was reading for a while. When Zack (Cloud in his story) went to the basement, he checked up of Sephiroth. A few days later the Shinra Troop (more than likely it was Cloud) told Zack that something was wrong with Sephiroth. Zack went downstairs, got a monologue by Sephiroth, was pushed aside, and then Sephiroth left the mansion. If he didn't killed the troop, I dunno. But what I soeculate is that CLoud was the troop, he went outside looking for Sephiroth when he heard screams, and burning villages, and was then attacked by Sephiroth or passed out from the smoke. As you saw, the troop laying in front of Cloud's house WAS Cloud, and Zack was the sane one. Then it goes to the Mako Reactor scene.